The compiler has to know that a function called "overDraft" exists before it gets to a line that calls that function. There are two options:

1) you can declare that the function exists by putting a "prototype" of the function before "withdraw". The prototype for overDraft has to exactly match the function definition (but parameter names can be omitted) as follows:

bool overDraft(int pin); //prototype of overDraft, to put before withdraw